One of the current state-of-the-art process for manufacturing VCM combines ethylene, hydrogen chloride and oxygen in the presence of copper chloride to form ethylene dichloride, which is decomposed at high temperature to form VCM and HCl. In an alternative process, ethyne (acetylene) from carbide is reacted with HCl over a mercury-based catalyst. This process is preferred where there is access to cheap ethyne via carbide from abundant coal resources. The mercury catalyst, usually 8-10% mercuric chloride on activated carbon, used in this process is highly toxic. Such toxicity creates problems arising from handling during manufacture of the catalyst as well as during loading of catalyst and removing catalyst after a campaign. A campaign is generally of six month duration. Deactivation of the mercury catalyst and loss of HgCl2 by sublimation or volatilisation from the reactor in use can lead to significant problems. The ethyne process using a mercury-based catalyst requires a lower capital investment in plant than the ethylene process. If a non-volatile and less toxic catalyst could replace the mercury catalyst without requiring significant plant alterations in existing plant designs, this would be a significant advantage.
There have been a number of academic studies and publications which recommend a gold-based catalyst in the ethyne-HCl reaction, as being the most active catalyst for that reaction. Deactivation of gold catalysts still occurs, although it has been said that high loadings (>1 wt %) of gold on carbon reduces deactivation. Other observations on the deactivation of gold catalysts have been that deactivation occurs both at high and at low temperatures, and that low temperature deactivation appears to be caused by coke formation, probably as a result of surface reactions of vinyl chloride and ethyne.
JP 522136104 in the name of Denki Kagaku Kogyo KK, and dating from 1977, teaches the use as a catalyst in the production of VCM, of a gold halide in combination with a platinum halide or a palladium halide, on a carbon support. The preparation of the catalyst is not described, and no commercial use of the process or the catalyst appears ever to have taken place. Conte et al, (Catalysis Letters, vol. 124, no. 3-4, 22 Jul. 2008, pages 165-167), describes the reactivation of a C-supported gold catalyst when used in the hydrochlorination of acetylene. The catalyst is prepared by incipient wetness impregnation of carbon, with a solution of HAuCl4 in aqua regia, followed by drying. Reactivation is by boiling in aqua regia. BonganiNkosiet al, (Journal Of Catalysis, Vol. 128, 1991, pages 378-386) describes hydrochlorination of acetylene using C-supported gold catalysts made by incipient wetness impregnation of an extruded carbon with a solution of HAuCl4 in aqua regia and reactivation with HCl in situ. Thompson (Gold Bulletin, vol. 31, no. 4, 1998, pages 111-118) discusses hydrochlorination of ethyne using gold catalysts made by impregnation of activated carbon with chloroauric acid dissolved in aqua regia.
There remains a need for a catalysed process for the production of VCM from ethyne that can be used in new or in existing plants using a catalyst which resists deactivation or at least has no worse lifetime, and has desirably a longer effective life, than the mercury catalyst, and which uses a catalyst that has little or no toxicity. WO2010/055341 describes a catalyst comprising gold nanoparticles carried on a carbon support which is active for the reaction of ethyne with hydrogen chloride to form vinyl chloride. The catalyst is made by impregnating an extruded carbon support with a solution of HAuCl4 in aqua regia, which is a concentrated mixture of hydrochloric and nitric acids. Use of aqua regia adds complexity to the manufacturing process because it is toxic and also highly oxidising, so that the manufacturing equipment must be protected from damage by the acid. Also the removal of excess acid results in the creation of HCl and NOx vapours which must be treated to avoid environmental pollution and to maintain a safe working environment. We have now found an improved catalyst and method for its manufacture which does not employ aqua regia as a process solution and which therefore avoids some of the problems associated with making the catalysts of the prior art.